Shock & Vibration Test Fixture Design

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Introduction

The best vibration fixtures are simple, rigid, lightweight with the minimum number of features.  Shaker and power amplifier systems are limited in terms of voltage, current and force pounds.  Lighter fixtures are need to achieve higher acceleration levels.

Materials

Aluminum alloy is a good choice for small to medium fixtures because it is cheaper than other materials such as magnesium.  Aluminum 6061-T6 & 7075-T6 are suitable alloys.

Magnesium is a better choice than aluminum in terms of weight to stiffness ratio and can provide higher damping.

Magnesium Zirconium K1A alloy in cast form has excellent hysteresis damping.

Magnesium Alloy AZ31B-Tool Plate also has very good damping.  It is a free machining material with good weldability properties.

Fixtures should have high damping in order to limit their resonant response.

Higher Damping also provides a transmissibility ratio relatively closer to unity gain across the entire frequency spectrum.

Joints

Some fixtures require joints. Bolted joints have more damping than welded ones. But welded joints have more rigidity if they are continuous.

Welded joints can be used with caution. They may have material distortion and high residual stress from thermal expansion and contraction during the welding process. After welding and before machining, the fixture should be left overnight in an oven at 250 °C to reduce warping and residual stress.

Welded joints may also have porosity, inclusions and microcracks. Some amount of microcracking is inevitable but needs to be managed via inspection and fracture calculations.

The quality of the weld is paramount. The weld joints should be continuous and smooth, with two or even three passes if possible.

References

Welded Joint Concerns for Shock, Vibration & Fatigue

Bruel & Kjaer Fixture Guidelines

See also:  Material Damping

– Tom Irvine

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